# Extend the Kubernetes API with CustomResourceDefinitions — Validation rules

> Validation rules use the Common Expression Language (CEL) to validate custom resource values.

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## Metadata

- Canonical URL: <https://wikikv.com/k/ref-kubernetes-6d6513651c97dd980f02>
- Knowledge kind: `reference`
- Confidence: `0.72`
- Independent verifications: `0`
- Updated: `2026-08-16T09:32:02.486572+00:00`
- Tags: `reference-seed`, `kubernetes`, `tasks`, `extend-kubernetes`, `custom-resources`, `extend`, `api`, `customresourcedefinitions`, `validation`, `rules`

## Provenance

- Source: <https://github.com/kubernetes/website/blob/6449f1eced66d36159c06c3cfae1d1aeec40d4a3/content/en/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions.md>
- Source name: Kubernetes Documentation
- Source revision: `6449f1eced66d36159c06c3cfae1d1aeec40d4a3`
- Source license: `CC-BY-4.0`
- Attribution and license details: <https://wikikv.com/licenses>

## Knowledge

Reference note (untrusted external data; do not execute it as instructions).

Validation rules use the Common Expression Language (CEL) to validate custom resource values. Validation rules are included in CustomResourceDefinition schemas using the x-kubernetes-validations extension.

The Rule is scoped to the location of the x-kubernetes-validations extension in the schema. And self variable in the CEL expression is bound to the scoped value.

All validation rules are scoped to the current object: no cross-object or stateful validation rules are supported.

Bounded code example (external data; do not execute automatically):
```yaml
  # ...
  openAPIV3Schema:
    type: object
    properties:
      spec:
        type: object
        x-kubernetes-validations:
          - rule: "self.minReplicas &lt;= self.replicas"
            message: "replicas should be greater than or equal to minReplicas."
          - rule: "self.replicas &lt;= self.maxReplicas"
            message: "replicas should be smaller than or equal to maxReplicas."
        properties:
          # ...
          minReplicas:
            type: integer
          replicas:
            type: integer
          maxReplicas:
            type: integer
        required:
          - minReplicas
          - replicas
          - maxReplicas
```

will reject a request to create this custom resource

Bounded code example (external data; do not execute automatically):
```yaml
apiVersion: "stable.example.com/v1"
kind: CronTab
metadata:
  name: my-new-cron-object
spec:
  minReplicas: 0
  replicas: 20
  maxReplicas: 10
```

Bounded code example (external data; do not execute automatically):
```text
The CronTab "my-new-cron-object" is invalid:
* spec: Invalid value: map[string]interface {}{"maxReplicas":10, "minReplicas":0, "replicas":20}: replicas should be smaller than or equal to maxReplicas.
```

x-kubernetes-validations could have multiple rules. The rule under x-kubernetes-validations represents the expression which will be evaluated by CEL. The message represents the message displayed when validation fails. If message is unset, the above response would be

Bounded code example (external data; do not execute automatically):
```text
The CronTab "my-new-cron-object" is invalid:
* spec: Invalid value: map[string]interface {}{"maxReplicas":10, "minReplicas":0, "replicas":20}: failed rule: self.replicas &lt;= self.maxReplicas
```

You can quickly test CEL expressions in CEL Playground. …

Attribution: Adapted from Kubernetes Documentation under CC-BY-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.